The primary attraction at Jewel Cave National Monument is calcite crystals. These sparkling jewel-like formations enticed Frank and Albert Michaud, the cave's founders, to stake a mining clam in 1900 and convinced the U.S. government to declare Jewel Cave a national monument 8 years later. While no riches were found by the Michaud brothers more than 146 miles (235 km) of maze-like passageways have been discovered by volunteer explorers. Jewel Cave National Monument is the second longest cave the world after Mammoth Cave in Kentucky. In addition to glittering calcite crystals, Jewel Cave boasts helictites, scintillites and frostwork formations as well as both stalactites and stalagmites. And it is home to 9 different kinds of bats.